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    Longitude around noon. was: Re: Navigation exercise
    From: George Huxtable
    Date: 2008 May 31, 14:33 +0100

    I have altered the thread name to make it more relevant.
    Bill has provided an interesting account of his path to enlightenment, and I
    go along with all he says, except for a little niggle at the end.
    His comment- "The vessel's motion would keep elevation climbing to an
    onboard observer until the rate of climb induced by the vessel cancelled out
    the rate of fall after LAN (to an observer at a fixed position)." shows that
    he has really understood the matter.
    Fuirther on, he wrote- "While I appreciate the Clyde problem you posted both
    for the prose and word pictures, it seemed to me that N 56 during the winter
    solstice would produce a pretty flat curve."  Well, that was its point, of
    course; to test Frank's pet proposal in a demanding (but realistic)
    situation, not just in a benign environment, to check the claims he has made
    for its accuracy.
    Bill's alternative situation gives that method a much easier ride, with the
    Sun's maximum altitude up around 73 deg. Then the curve of altitude against
    time would not be nearly so flat as in that Clyde-approach, which makes the
    timing much less imprecise. In Jeremy's recent observations from the
    tropics, the Sun is even higher, and Frank's longitude-around-noon method
    may then be perfectly appropriate.
    Even so, from 40 North in midsummer, with a Southerly speed of 12 knots,
    Bill concludes-
    "calculated latitude (no instrument or observer error) will only be shifted
    by 0d 00!2 towards north (plot away). Time of peak altitude vs. LAN is late
    by *nominally* 1.5 minutes. That equates to 23 nm, or 0d 29' lon at N 40.
    That's a ton to me, especially as it is a theoretical and does not include
    other potential errors."
    I haven't checked the details of Bill's calculation, but at 4 minutes of
    time to a degree (the speed at which the World spins), his deduced
    time-shift of 1.5 minutes would correspond to a longitude shift, if
    uncorrected, of 22.5 minutes of arc, or at 40 deg North, a distance error of
    17.2 miles. So a bit less than his own figure of 23nm, then; not quite a
    "ton", perhaps, but still a serious matter, not to be taken lightly.
    Bill refers to my words- "But much of this is old ground, which has been
    well-trodden on this list before, and I doubt whether any minds are going to
    be changed."
    and asks-
     "References to the "old ground" please. Thread name plus month a help."
    To be honest, I would much rather leave it at that, rather than reopen
    further fruitless debate, between the zeal of a prophet who claims to have
    discovered a new way to teach navigation, and the contrary view that
    students should discover navigation via position lines.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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